The simplest fuel cell comprises two electrodes separated by an electrolyte. The electrodes are electrically connected through an external circuit, with a resistive load lying in between them. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly, or xe2x80x9cMEA,xe2x80x9d comprising a solid polymer electrolyte membrane, or xe2x80x9cPEM,xe2x80x9d also known as a proton exchange membrane, disposed between the two electrodes. The electrodes are formed from porous, electrically conductive sheet material, typically carbon fiber paper or cloth, that allows gas diffusion. The PEM readily permits the movement of protons between the electrodes, but is relatively impermeable to gas. It is also a poor electronic conductor, and thereby prevents internal shorting of the cell.
A fuel gas is supplied to one electrode, the anode, where it is oxidized to produce protons and free electrons. The production of free electrons creates an electrical potential, or voltage, at the anode. The protons migrate through the PEM to the other electrode, the positively charged cathode. A reducing agent is supplied to the cathode, where it reacts with the protons that have passed through the PEM and the free electrons that have flowed through the external circuit to form a reactant product. The MEA includes a catalyst, typically platinum-based, at each interface between the PEM and the respective electrodes to induce the desired electrochemical reaction.
In one common embodiment of the fuel cell, hydrogen gas is the fuel and oxygen is the oxidizing agent. The hydrogen is oxidized at the anode to form H+ ions, or protons, and electrons, in accordance with the chemical equation:
H2=2H++2exe2x88x92
The H+ ions traverse the PEM to the cathode, where they are reduced by oxygen and the free electrons from the external circuit, to form water. The foregoing reaction is expressed by the chemical equation:
xc2xdO2+2H++2exe2x88x92=H2O 
Solid Oxide Fuel cells (SOFCs) operate using a mechanism similar to PEMs. The main difference is that instead of the electrolyte material comprising a polymer material capable of exchanging protons, the electrolyte material comprises a ceramic material capable of exchanging electrons.
Electrode layers must be porous in order to allow the fuel and oxidant to flow to the electrode-electrolyte interfaces. Typical fuel cells that use porous electrode materials are bulk structures that require significant manifolding and pressures to readily deliver the fuel to the electrode-electrolyte interface. These porous electrodes are formed by pressing and sintering metal powders to promote adhesion, then sandwiching two such electrodes around an electrolyte layer to form a fuel cell or in series to form the fuel cell stack. A method to fabricate porous electrodes that can reduce or remove the need for high temperatures or high pressures to assist the flow of the fuel and oxidant to the electrode-electrolyte interface may be an important contribution to fuel cell technology.
Aspects of the invention include a method comprising the steps of
Simultaneously: (1) coating a topside of a porous host structure with a plurality of conductive material particles and (2) flowing gas through a bottom side of the porous host structure to form a conductive porous electrode layer on the topside.
Other aspects of the invention include an electrode comprising a conductive material having a plurality of pores, the electrode having a pore size distribution wherein at least 90% of the total pore volume is in pores of diameter from about 10% below the mode pore diameter to about 10% above the mode pore diameter.
A fuel cell comprising at least one electrode comprising a conductive material having a plurality of pores, the electrode having a pore size distribution wherein at least 90% of the total pore volume is in pores of diameter from about 10% below the size of the mode pore diameter to about 10% above the size of the mode pore diameter.
A fuel cell stack comprising at least one fuel cell having at least one electrode comprising a conductive material having a plurality of pores, the electrode having a pore size distribution wherein at least 90% of the total pore volume is in pores of diameter from about 10% below the size of the mode pore diameter to about 10% above the size of the mode pore diameter.